The goal of this experiment was to investigate the effects of dietary protein levels on muscular amino acids and protein metabolism of Regan. Six diets were formulated that contained 20%, 25%, 30%, 35%, 40%, and 45% protein levels. Each diet was fed to triplicate groups of 30 fish with an initial body weight of (22.42±0.56) g for 60 d. The results showed that both nitrogen intake (NI) and the amount of absolute nitrogen intake (ANI) of Regan increased with the increase in dietary protein levels. The nitrogen deposition (ND), protein efficiency ratio (PER), and net protein utilization (NPU) increased at first and then decreased. Total essential amino acids (TEAA), total flavored amino acids (TFAA), total non-essential amino acids (TNEAA), and total amino acids (TAA) initially increased and then tended to stabilize with the raising supplemental dietary protein level. There was no significant difference in blood ammonia, urea nitrogen, or albumin (ALB) at the feed protein levels of 20%-25%; however, these factors increased significantly after the feed protein level increased significantly after 30%. Total protein (TP) increased gradually when feed protein was lower than 35%, and there was no significant difference among the experimental groups when feed protein was higher than 35%. Both the liver ALT and AST increased at first and then tended to stabilize. There was a tendency for serum ALT to decrease first and then increase. Serum AST was not significantly different in the experimental group with feed protein content below 35% but increased significantly above 35%. Thus, the appropriate amount of dietary protein level can increase the absorption of protein, increase the protein utilization from feed, increase the rate of body protein metabolism, improve the content of essential amino acids, and improve the taste of muscle. However, it can cause a decrease in the protein utilization in the fish body, inadequate absorption of the protein in the intestine, the increase nitrogen excreta, aggravate the liver load, and damage liver cells when the content of feed protein is too high. Under these experimental conditions, the suitable dietary protein level was 30%-35% based on the feed protein utilization, total protein content in blood, and liver and serum transaminase activity.